Geographic data and visualization
GEOG 30323
November 17, 2015
What is geographic data?
- Thus far: we’ve been working with data (largely) in tabular data frames, where each column represents data “attributes”
- Geographic data: includes information on location - specifically, where the observation is located on the Earth’s surface
Longitude and latitude: the basics

Geographic coordinates
- Longitude (X) and latitude (Y) coordinates can be expressed as:
- Degrees minutes seconds (e.g. \(97^{\circ}21'37''W\), \(32^{\circ}42'38''N\))
- Decimal degrees (e.g. \(-97.36\), \(32.71\))
Conversion between DMS and decimal degrees:
\[DD = D + \frac{M}{60} + \frac{S}{3600}\]
Coordinate systems
- Geographic coordinate system: coordinate system based on latitude and longitude (coordinates on a sphere)
- Projected coordinate system: coordinate system projected onto a two-dimensional surface (coordinates on a plane)
Spheres and planes
Source: Mike Bostock
Map projections
Original source: Mike Bostock
Tiled mapping and Web Mercator

Vector data

Raster data

Vector data: the shapefile
- Shapefile: common format for encoding vector geographic data
- Three required files: .shp, .dbf, and .shx; .prj recommended
Maps vs. charts
- For discussion: which visualization is more effective for showing differences in our data?
Chart for comparison
